Spinal fusion is indicated to provide stabilization of the spinal column for disorders such as structural deformity, traumatic instability, degenerative instability, and post resection iatrogenic instability. Fusion, or arthrodesis, is achieved by the formation of an osseous bridge between adjacent motion segments. This is accomplished either anteriorly between contiguous vertebral bodies or posteriorly between consecutive transverse processes, laminae or other posterior aspects of the vertebrae.
Typically, the osseous bridge, or fusion mass, is biologically produced by recreating conditions of skeletal injury along a "fusion site" and allowing the normal bone healing response to occur. This biologic environment at a proposed fusion site requires the presence of osteogenic or osteopotential cells, adequate blood supply, sufficient inflammatory response, and appropriate preparation of local bone.
Under conventional wisdom, it has been generally understood that decortication is required to prepare the bone to increase the likelihood of fusion. Decortication is the processing of removing the outer cortex of spinal bone with a burr to induce bleeding bone and release of its bone marrow contents. Decortication also initiates the inflammatory response, releases osteoinductive cytokines, provides additional osteogenic cells, and creates a host attachment site for the subsequent fusion mass. However, decortication increases blood loss, operative exposure, operative time, post operative pain and the chance of neurological damage. According to a recent study, decortication was deemed essential for fusions without segmental instrumentation although it may not be necessary for fusions with internal fixation. (Ishikawa, Spine: 1686-1690, 1994.)
Bone graft materials are often used to promote spinal fusions. Although autogenous iliac crest cortico-cancellous bone is presently the most successful bone grafting material, the rate of fusion mass consolidation with use of autograft is limited by the rate of normal biologic bone repair. Autograft also requires an additional surgery which increases the risk of infection and may reduce structural integrity at the donor site. Furthermore, many patients complain of significant pain for several years after the surgery. These disadvantages have led to the investigation of bioactive substances that regulate the complex cascade of cellular events of bone repair, such as bone morphogenic protein, for use as alternative or adjunctive graft materials. Bone morphogenic protein (BMP), an osteoinductive cytokine extracted from bone matrix, is capable of inducing bone formation when implanted in a fracture or surgical bone site. BMP actually refers to a group of bone morphogenic proteins belong to the TGF-.beta. super family. The structures of eleven proteins, BMP-1 through BMP-11 have been elucidated. Recombinantly produced human bone morphogenetic protein-2 (rhBMP-2) has been demonstrated in several animal models to be effective in regenerating bone in skeletal defects.
The following events are thought to occur when the BMPs are applied to a bony site: Osteogenic and chondrogenic precursor cells accumulate, cartilage forms and matures, and vascularization occurs during bone formation. As bone formation proceeds, the cartilage and carrier are resorbed. The final result is the restoration of bone and bone marrow in the defect site. The purification of bovine bone-derived bone-inductive protein (Wang et al. 1988) led to the cloning of recombinant human (rh) BMP-2 through rhBMP-8 (Wozney et al. 1988; Wozney 1989; Celeste et al. 1990; Celeste et al. 1992). BMP-2 through BMP-8 are related proteins with several common characteristics. Each BMP is synthesized in a precursor form, with a hydrophobic secretory leader sequence and a substantial propeptide region. The mature protein consists of a dimer of the carboxy-terminal portion of the propeptide molecule. All of the mature regions of these rhBMPs contain one or more N-linked glycosylation sites and seven cysteine residues. The locations of the cysteine residues are conserved within all members of this gene family. The BMPs are proving useful in spinal surgeries in promoting bone healing.
In spite of these developments in bone graft materials, conventional wisdom still maintains that decortication of the fusion site is essential to complete osteoinduction and osteogenesis. Consequently, the above-mentioned drawbacks and risks remain in many fusion procedure, whether or not instrumented with fixation implants. A need has remained for surgical spinal procedures which do not require decortication.